As shown in Fig. 1, the chip which we made comprises a multilayer chip structure in the size of 2.0 x 1.2 x 1.2mm3 consisting of SrTiO3 ceramics added with 0.8 atomic % of La and Pd internal electrodesand Ag external electrodes. The chip initially showed voltage nonlinearity where a current flowed suddenly by applying voltage, as can be seen in SrTiO3 type varistorsafter forming process was made to apply high voltages under the limited current of 100mA, a large hysteresis was observed in the current-voltage characteristics as shown in Fig. 2. When voltage swept from 0V to -150V, the current suddenly ran at -80V, switching to the low resistance state from the high resistance state. When the voltage swept from 0V to +150V, negative resistance, which slowed down the flow of current, gradually started to appear, thereby switching to the high resistance state again from the low resistance state. Applying voltages of different polarities made it possible to switch reversibly from low resistance state to high resistance state and vice versa. The resistance changes were fairly large in about three digits at low voltages. This resistance change behavior upon applying voltage is called “bipolar type CER characteristics”; being ceramics, a large resistance change ratio equivalent to the one held by a thin film ReRAM was achieved.
Fig. 1 Structural pattern diagram of a chip made by the research team and cross-section image by field emission-type scanning electron microscope
Fig. 2 Typical current-voltage characteristics of a prototype chip
a: Linear scale b: Semilog scale
- Colossal electroresistance effect (CER):
When voltage is applied, a large resistance change in one digit or more appears, having an effect that its resistance state can be memorized. A bipolar type has a phenomenon where a resistance state changes when reversible voltage is applied, and a non-polar type or unipolar type has a phenomenon where a resistance state changes when the same polarity voltage is applied.
- ReRAM (Resistance Random Access Memory):
A kind of nonvolatile semiconductor memory making use of the CER phenomenon